The Redo function is the reverse of the Undo function. Redo undoes an undo — if you have painted a line with the Paintbrush and then clicked Undo to remove it, you can make it reappear by clicking Redo.
As with the Undo function, you may want to use the Undo History (Image Menu Dialogs Undo History...) which allows you to make multiple Undo and Redo operations at once.
Default Keyboard Shortcut: Ctrl-R